From the left side of the heart, blood full of oxygen is pumped into the systemic circulation. This blood leaves the heart through the aorta, the main artery of the body. A number of major arteries branch off the aorta. These arteries, in turn, branch into smaller and smaller vessels, finally emptying into the tiny capillaries. There, oxygen leaves the blood and enters the tissues through the thin capillary walls. In a similar way, carbon dioxide leaves the tissues and enters the blood. The blood, now carrying carbon dioxide, leaves the capillaries and flows through larger and larger veins. Eventually, the blood enters the right side of the heart through two large veins--the superior vena cava, which carries blood from the head and arms, and the inferior vena cava, which carries blood from the trunk and legs.
From the right side of the heart, the blood is pumped into the pulmonary circulation. Pulmonary arteries carry the blood that contains carbon dioxide to capillaries in the lungs. The carbon dioxide passes through the capillary walls into the lungs and is then exhaled. Oxygen that has been inhaled passes from the lungs into the blood in a similar way. The blood returns through the pulmonary veins to the left side of the heart and begins its journey again.